1. Field of the Invention
The present invention relates to a process for irradiating a target material, and in particular, to a process for irradiation producing a constant dose of radiation at various depths within the irradiated material.
2. Description of the Related Art
Controlled irradiation of target materials is a mature technology having many industrial applications. Important uses for irradiation include lithography in the fabrication of semiconductor devices, high-power magnification and imaging in the form of electron microscopy, cross-linking of polymeric materials, and sterilization of medical devices and foodstuffs.
Each of these applications involve the generation of radiation from a source, followed by direction of this radiation to a target material. Emission of a variety of forms of radiation is commonly utilized, including electron beam, x-ray, and gamma radiation.
Conventional irradiation processes suffer from an important disadvantage in that the dose of radiation delivered to an irradiated object varies over the thickness of the target material.
FIG. 1 shows a typical depth/dose profile resulting from exposing a target material to conventional electron beam irradiation. FIG. 1 shows that the relationship between radiation dose and material depth is nonlinear. For example, the radiation dose is lower at the surface of the target material than at a depth X into the target material. In a conventional method of electron beam irradiation, the peak subsurface irradiation dose can be as much as 30-50% greater than the surface dose.
While FIG. 1 depicts the depth/dose profile for electron beam irradiation, both x-ray and gamma radiation also exhibit a profile similar to that shown in FIG. 1.
For electron beam irradiation, the non-linear character of the curve shown in FIG. 1 is attributable to the impact of high energy radiated electrons with low energy local electrons present in target surface regions. The initial impact of these high energy electrons with local surface electrons imparts energy to the local electrons, which then penetrate more deeply. The penetrating electrons in turn collide with local electrons positioned even more deeply within the target, displacing them further into the target material.
As a result of this chain reaction, the impact of high energy electrons at the surface results in the shifting of maximum radiation concentrations to subsurface regions. However, below a depth X' in the target material, energy imparted to the target material becomes sufficiently diffused that local electrons no longer possess sufficient energy to penetrate further, and the radiation dose tails off.
This nonlinear relationship between radiation dose and target material depth creates a number of problems. One problem is lack of predictability. Because of the nonlinear depth/dose relationship, in order to anticipate the expected radiation dosage engineers must resort to statistical computer programs utilizing Monte Carlo approximations. These approximations are complex, time consuming, and costly.
Therefore, there is a need in the art for a method of irradiation that provides a linear relationship between electron dose and the thickness of the irradiated material.
An even more important problem with conventional irradiation techniques is that subsurface regions can be expected to receive heavier doses of radiation than surface regions. For example, where electron beams are applied to trigger polymerization and cross-linking, the dose profile shown in FIG. 1 can lead to an uneven degree of polymerization and hardness at different depths within the material. This non-uniformity of cross-linking can create quality control and other problems. Similarly, where electron beams are applied to sterilize a material, variation of dose with depth can lead to nonuniform sterilization and the possibility of infection and other problems.
In theory, the problem of variation in radiation dosing can be overcome by applying such intense radiation that even surface material regions receive sufficiently high doses. In practice however, this approach can cause a host of problems associated with over-irradiation of the subsurface regions.
Perhaps most significantly, subsurface regions receiving heavier doses of radiation can begin to degrade. Moreover, accumulated heat from the over-irradiation can also affect temperature-sensitive target materials such as plastics or foodstuffs. In addition to problems with degradation and heat, excess electron beam irradiation needlessly consumes large amounts of power and imposes strain on expensive and difficult-to-maintain irradiation equipment.
Therefore, there also is a need in the art for a method of electron beam irradiation that produces a relatively constant dose of electrons from target surface regions to subsurface target regions.